Analog circuits such as those employed in analog-to-digital, digital-to-analog converters and in speech synthesizers make extensive use of current source polarity switching circuits. Such circuits, often called "current source polarity switches", are capable of producing constant current flow alternating in opposite directions. For example, when the input of the switch receives a control voltage at a certain level, its output will source a current to the load and when the input of the switch receives a control voltage at another level its output will sink constant current from the load.
Current source polarity switching circuits can be readily fabricated from transistors. The metal-oxide semiconductor field-effect transistors (MOSFETs) are widely used for large scale integrated circuits because of their low power consumptions and ease of manufacture. So called CMOS current source polarity switching circuits are those made up of opposite channel type metal-oxide semiconductors. Such circuits utilize two P-channel MOS transistors and two complementary N-channel MOS transistors with two of the transistors acting as current sources and two of the transistors acting as switches. One of the P-channel MOS transistors of the CMOS current source polarity switch circuit is used to supply source current to the output through the other P-channel MOS transistor which functions as a switch. And one of the N-channel MOS transistors of the CMOS current source polarity switch is used to supply sink current to the output through the other N-channel MOS transistor which also functions as a switch. A variable voltage control signal impressed on the input of the CMOS transistor switches causes one MOS transistor to turn on as the other MOS transistor is turned off and vice versa, depending on the voltage level at the input.
This CMOS current source switching circuit frequently is used in combination with other transistors which are employed to bias the amount of current in the sourcing and sinking transistors.
It has been observed that current source switch circuits of the type described above often do not produce a precise two level signal output. Rather, they produce an output signal which exhibits a current overshoot or "spike" from the transistor being turned on. Such spikes constitute extraneous signals or "noise" in the output signal and are very undesirable. Any increase in the noise to signal ratio of the polarity switching circuit adversely affects the quality of the integrated circuit embodying the circuit.
There has been a need, therefore, for improved circuitry to reduce the spike noise in current source polarity switching circuits.